Chinchilla an aggressive biter?

Chins like rabbits rarely bite. When you get one that does it's usually as you described. I have bred and rescued chins for eleven years now and only had it happen once with one I raised. Neutering is worth a try and you could ask for a check up to rule out a medical means as cause at the same time. One other thing you might try is to give them both their dust bath in another cage or area. That way the do get a reward for coming out of the cage. Just don't make it to comfortable that they don't want to go back. When it becomes routine they look forward to coming out. Your bf is doing all the right things and sounds like a good animal handler. While the biters could be a inherited gene there could be other factors that are at work psychologically or physically. I have had only one young one less than seven month do to me exactly what you described. That is one of several hundred chins that have passed through my hands. I have also had it happen to me at another much larger breeder that I know and also witnessed her getting bit as well.